function lazyLoad(fn, threshold = 100) {

    let flag = true; //开关是开启的

    window.onscroll = function() {
        //获取关键属性 

        let scrollTop = document.documentElement.scrollTop;

        let clientHeight = document.documentElement.clientHeight;

        let scrollHeight = document.documentElement.scrollHeight;


        let scrollBottom = scrollHeight - (scrollTop + clientHeight)

        if (scrollBottom <= threshold) {
            if (flag) {
                flag = false;
                fn()
            }
        } else {
            flag = true;
        }
    }
}